Now you are the lucky owner of a water garden or backyard pond, you may have always intended that this would be a home for fish. But what fish? 

Apart from ordinary Goldfish, fish that are suitable for average pools of any size include: Red Comets (Sarasas), Shubunkins, Golden Orfe, Tench and Rudd. These will all happily live with one another. More ornamental cold water Goldfish such as fantails and bubble-eyes, find it difficult to compete with other types particularly in the colder months. It is probably best to 'over winter' them in an aquarium indoors if you want to keep them There are always exceptions to the rule and here in one of my first ponds, koi do live in a tolerable harmony with plantssafe and healthy.
Various forms of ‘Fantails’, being slower movers than many pond fish, would find it difficult to compete in winter 
All the above fish are quite content to share their lives with Koi Carp. However serious keepers of Koi Carp need to think in terms of a minimum depth of 3ft - preferably 5 to 6ft - with a proportionately large width and length.
Koi keepers very often have to think in terms of no plants or at least have them protected from the vandalism of the fish. Therefore because of the lack of the biological cycle in which plants form an essential link, and also because of the rapid metabolism of these potentially very large fish, a sophisticated filter system is essential in keeping Koi. They are not fish of extremely cold water and find our winters stressful to say the least. When fish are stressed, then they are open to disease and parasites, and since Koi are such expensive fish, the only way to keep them with confidence is to employ all the quirks of pool design and filter technology at your disposal. (See all the recent biofiltration articles)

But first, before you even think of introducing any fish to your pond.......

POND HEALTH AND THE TECHNOLOGICAL SAFETY NET: beginners guide to the myriad of filters, biological filters, ultra violet  clarifiers, bubble bead filters, skimmers and protein skimmers etc.

The picture above demonstrates in a simple form the natural cycle of
organic substances (represented by the red arrows) being broken down in a pond with a biological
filter. This is called the Nitrogen cycle and in this case
demonstrates in particular what happens to fish food as it is digested
first by the fish and then the resulting faeces are then processed by
bacteria in the filter medium. The cartoon is one that featured in 'The Perfect Pond Detective Book 1: The biological balance' by Peter May, available from this site.

Biological filtration:

is a method of removing detritus/muck, organic compounds and specifically ammonia and nitrites from pond water by pumping pond water into a container and straining it through various foams, meshes or filtration media in which there is an established population of specific bacteria. These bacteria need plenty of oxygen to survive and they use it to digest organic compounds and a lot of the sediment that would otherwise lie decomposing in the same way on the bottom of the pond, although much more slowly. For small ponds there are two basic types of designs of filter that are built to work like this: Gravity filters and Pressure filters.